Silent tuning radio receiver



Feb. 4, 1941. H. c. TITTLE SILENT TUNING RADIO RECEIVER Filed Sept. l5,1937 MZHJ ZMDHHM nim@ Hunsm' CLTrrT'LE LA L@ @zmsmwwww Patented Feb. 4,1941 barro STATES PATENT OFFlCE SILENT TUNING RADIO RECEIVER ApplicationSeptember 15,` 1937, Serial No. 163,947

7 Claims.

This invention relates to improved so-called silent tuning systems forradio receivers, i. e., tuning systems in which the receiver is soconstructed and arranged as to require a signal of predeterminedstrength to cause response, and which may be so adjusted that signalsvof less than such strength, atmospherics, man-made static, etc., willnot produce a response. With such a receiver, it is possible to tunefrom one station to another having the predetermined strength, and havethe receiver quiet during the time of transition between stations,whereas, the usual type receiver embodying so-called automatic volumecontrol is most sensitive when no signal is being received, and islikely to be very noisy when being tuned from one station to another.

The usual type silent-tuning receiver, however, is open to the objectionthat in order to keep the receiver in responsive condition, the signalstrength must be at all times equal to or greater than that required tocause initial operation. This is particularly troublesome in automobilereceivers, in which the signal strength is likely to iiuctuate as thevehicle travels, even in the case of strong signals, and reception maythereby be interrupted from time to time.

It is `an object of this invention to pro-vide a receiver of theso-called silent tuning type, which is relatively free from thedrawbacks heretofore associated with such receivers.

More particularly, it is an object of this invention to provide areceiver in which a predetermined incoming signal strength is requiredto cause response of the receiver, but which when once set intooperation by a signal of such strength, will nevertheless continue torespond to such station even if the strength of the signal receivedshould fall far below the level required to cause initial response.

It is a further object of this invention to provide a silent tuningreceiver embodying automatic volume control, which requires a signal ofpredetermined strength to cause it to be responsive, but which, oncerendered responsive by a signal of such strength, will thereafteroperate like an automatic volume control receiver, as long as thestation continues to be received, even though the strength of the signalreceived drops far below that required to produce initial response ofthe receiver.

It is a further object of this invention to provide a receiver whichrequires an incoming signal of predetermined strength to produce initialre- (ol. 25o-20) sponse, land which, once rendered responsive by asignal of such strength, will continue to respond to such signal eventhough it should drop far below the strength required to produce initialresponse, but which will cease to respond to the incoming signal if itfalls below a predetermined minimum strength level.

It is a further object of this invention to provide a receiver whichrequires an incoming signal of predetermined strength to produce initialresponse, and which, once rendered responsive by a signal of suchstrength, will continue to respond to such signal even though it shoulddro-p far below the strength required to produce initial response, andwhich will cease to respond to the incoming signal if it falls below apredetermined minimum strength level, and thereafter will beunresponsive to signals having less than the predetermined strengthrequired for initial response.

Still other objects and advantages of my invention will be apparent fromthe specication.

The features of novelty which I believe to be characteristic of myinvention are set forth with particularity in the specication. Myinvention itself, however, both as to its fundamental principles and asto its particular embodiments, will best be understood by reference tothe specification and accompanying drawing, in which:

Figure l is a .schematic circuit diagram of one form of circuitoperating according to my invention, and

Fig. 2 is a curve illustrating the operation of my invention.

The operation of a receiver according to my invention is somewhatanalogous to the welll known hysteresis eiect. Referring now moreparticularly to Fig. 2, the abcissae represent the strength of theincoming signal in micro-volts,

while the ordinates represent the strength of the signal output from thereceiver in volts. The direction to be followed around the loop is shownby the arrows.

From this it will be seen that so long as the incoming signal is lessthan 1,000 microvolts, no signal output will be produced; i. e., thereceiver will be unresponsive to incoming signals. Once the strength ofincoming signals reaches 1,000 microvolts, however, the signal output ofthe receiver increases extremely rapidly to the value indicated, in theneighborhood of 1.0 volt.

If for any reason the strength of the incoming signal should nowdecrease, the receiver will continue to respond, even though thestrength of the signal should drop below that'required to pro-v strengthreaches the value necessary to cause initial response; i. e., 1,000microvolts.

Referring now to Fig. 1, I have shown so much of the circuits of asuper-heterodyne receiver employing my invention as is necessary to anunderstanding of one preferred embodiment thereof. In this figure, theincoming signals may be received upon an antenna of any suitable type,not shown, and impressed upon the primary 50a of the high frequencytransformer 50, and thus upon the secondary 50D thereof.

The coil 50h, tuned by condenser 5I forms a tuned circuit tuned to thefrequency of the incoming signals, which -may then lbe amplified withoutchange of frequency in tube 52, herein lshown as a pentode embodyingheater 52a, cathode 52h, control grid 52c, screen 52d, suppressor 52e,and anode 52j. The cathode may be connected to ground throughself-biasing resistor 54 shunted by a by-pass condenser (not numbered)and the common point of coil 501) and condenser 5I may be connected tothe control grid, and the opposite terminals of each may be grounded,condenser 5| directly, and coil 50h through by-pass condenser 53. Theother electrodes of tube 52 may be connected as well known in the art. i

The anode 52j may be connected to a suitable positive point such as tothe positive side of the power supply, herein illustrated as a rectifierand filter system for operating from alternating current, well known inthe art, and not described in detail.

Included in the plate or anode circuit of anode 52f there may beprovided the primary winding 55a of the radio frequency transformer 55,the secondary winding 55h of which may form, with variable condenser 56,the tuned input circuit to the next tube 51, which may be, and is hereinillustrated as a penta-grid converter,

Tube 51 may comprise heater 51a, cathode 51h, various electrodes termedgri (although some of them may not be in conventional grid form) 51c,51d, 51e, 51f, 51g, and anode or plate 51h. 'I'hese grids may beinterconnected in the well known manner to cause the tube to generateheterodyning oscillations of the desired frequency and to combine theseoscillations with the incoming signal within the tube, the signals beingimpressed upon control grid 51o, from condenser 56.

Condenser 56 may have one side grounded, and the free end of coil 55hmay be connected to the grid return line, and may be separated from thereturn connection of grid 52o by resistance 14.

Cathode 51h may be connected to ground through self-biasing resistance,shunted by a by-pass condenser. The plate or anode 51h may be connectedthrough the primary winding 59a of intermediate frequency transformer 59to a suitable positive po-int on the power supply.

As will be understood the action of the tube is l632) throughresistances 64 and 65 to deliver to transformer 59 currents of beat orintermediate frequency, and it will be understood that the heterodyningoscillations may be generated in a separate tube, if desired, andcombined with the signals in any suitable manner. For this reason thespecific connections of the remaining electrodes of the tube .51 areomitted for simplicity, they being well known in the art.

The intermediate frequency signals in the secondary 5917 of theintermediate frequency transformer 59, both windings of which may betuned by trimmer or tuning condensers may be impressed upon the controlelectrode 60e of tube 60, which may still further amplify them atintermediate frequency. The cathode 60h may be connected to groundthrough a self-biasing resistance shuntedvby a by-pass condenser asbefore.

The return side of coil 59h may be connected to the grid return line,but separated from the connection of coil 55h by resistance 15, and bothlatter grid returns may be by-passed to ground through condensers 19 and80.

'Ihe plate or anode 60j may be connected through primary 61a ofintermediate frequency transformer 6| to a suitable positive point onthe power supply.

One terminal of the secondary Gib may be connected to the diode anode62o of the tube 62, herein shown as a diode-triode, and comprisingheater 62a, cathode 62h, diode anode 62o, control grid 62d, and main ortriode anode 62e. The output from the triode anode 62e may be audiofrequency currents only, which may be led to additional amplifyingstages, well known in the art, and not shown, and thence to a signalindicator, for instance a loud speaker, likewise not shown.

The opposite terminal of secondary 6| b may be connected -throughcondenser 13, forming by pass for radio frequency. A direct current pathfrom diode anode 62e to cathode 62D may be provided by resistance 12 andresistance 61 in series to ground, thence to cathode B2b, which may bedirectly grounded.

Resistance 6 1 may be shunted by condenser 69 and resistance 68 inseries, and Iaudio frequency currents may be supplied to lgrid 62d by aconnection through condenser 10 to a variable point on resistance 68.turned to any suitable point through resistance 1l, as is well known inthe art.

Control tube 63 may be provided, which may be of any suitable type,herein shown as a' diode triode, having heater 63a, cathode 63h, diode'anode 63o, control grid 63d, and triode anode 63e. Triode anode 63e maybe connected to the common point of resistance 61 and condenser 69, andthrough resistance 16 to the return side of' coil 5917. A

Diode anode 63e may be connected to cathode 1n series, the latter beingshunted by by-pass condenser 66, and its point of connection toresistance 64 may be connected through condenser 8lv to anode 60j.Cathode 63h maybe connected to the negative side of the power supply,and through variable tap 18 on resistance 11 to ground.

The operation of this circuit is as follows: for no signal, platecurrent flowing from cathode to triode anode of control tube 63maintains the triode anode 63e negative with respect to ground, thespace current path being from cathode 63h to tap 18, resistance 11 toground, through resist-i Grid 62d may be ref ance 61 to anode 63e. Sincegrids 52o, 51e, 66e, land diode anode 62o are all connected to triodeanode 63e, negative potential is applied to said electrodes also.

It will be clear that by regulating the strength of this bias voltage,as by adjusting the position of tap 13, sufcient bias may be maintainedon the tubes 52, l, Bil and 62, to prevent any response of the receiverexcept to signals of sufficient strength to override the bias, and whichstrength may be predetermined, and adjusted for.

If a signal exceeding this strength is received, current will begin toflow in the said tubes, and intermediate frequency current |appears inthe output circuit of tube 6D. Through condenser 3|, this or a portionof it is applied to the diode anode of control tube 63, and thereinrectified. Rectied current iiows from cathode 6319 through resistance65, and resistance 64 to diode anode 63e, thereby making controlelectrode 63d negative with respect to cathode 63h.

This causes the plate current in tube 63 to be cut ofi, and permits thegrids 52o, 51e, 60C, and diode anode 52e to assume a potentialdetermined by the rectified signal current of diode anode 62e, flowingthrough its space current path already traced, and which causes grids52e, 57e, and c, to become more negative with respect to their cathodes,thus tending to maintain a constant output from the diode portion oftube 52, regardless of the strength of the incoming signal, provided itsstrength lies between the limits at which blocking and unblocking occur.

As the signal strength decreases, the receiver will neverthelesscontinue to respond, until the signal falls to a strength far below thatrequired to unblock the receiver initially. For instance, if 1000microvolts is required to unblock the receiver and make it responsive,the receiver will continue to operate until the signal fades to 10microvolts, for example.

Should the signal strength fall below this value, the rectiedintermediate frequency currents flowing through resistance S5 will nolonger be suflicient to keep plate current from flowing in the triodeanode circuit of tube 63, and flow of this current will make anode 63enegative with respect to ground, and apply negative bias to grids 52o,57e, Silo, and diode anode 62o, returning to receiver to blocked, or nosignal condition.

The action is a sharp snapping over from blocked to unblocked condition,and vice versa, and in the hysteresis effect, i. e., the requirement ofa strong signal to cause unblocking, but once unblocked, the fading ofthe signal to a very much lower value to cause blocking to occur.

While I have shown and described certain preferred embodiments of ourinvention, it will be understood that modifications and changes may bemade without departing from the spirit and scope of my invention.

I claim:

l. In radio receiving apparatus, in combination, a plurality ofamplifier tubes connected for progressive amplification of incomingsignals, a thermionic rectifier supplied by said amplifier tubes, aconnection between said rectifier and the control electrode of at leastone of said amplifier tubes for 'applying rectified voltage thereto as abias to regulate the gain inversely in accordance with the strength ofincoming signals, a control tube supplied by the output of one of saidamplifier tubes, an impedance in the plate circuit of said control tubewhereby the plate potential thereof decreases with increase of spacecurrent in said tube, and a connection from said plate to the controlelectrode of at least one of said .amplifier tubes, and to saidrectifier for applying a blocking bias thereto.

2. In radio receiving apparatus, in combination, at least one amplifiertube connected for amplification of incoming signals at super-audiblefrequency, a thermionic rectier supplied by said amplifier tube, aconnection between said recter and a control electrode of said amplifiertube for applying rectified voltage thereto asa bias to regulate thegain thereof inversely with the strength of incoming signals, a controltube comprising an anode, a cathode and a control electrode, aconnection from said control electrode of said control tube to theoutput circuit of said amplifier tube, an impedance in the plate circuitof said control tube whereby the plate potential thereof decreases withincrease of space current in said control tube, and a connectionfromsaid anode to said rectifier for applying a blocking bias thereto.

3. In radio receiving apparatus, .in combination, at least one amplifiertube connected for amplification of incoming signals at super-audiblefrequency, a thermionic rectifier supplied by said amplifier tube, aconnection between said rectifier and a control electrode of saidamplifier tube for applying rectified voltage thereto as a bias toregulate the gain thereof inversely with the strength of incomingsignals, a control tube comprising an anode, a cathode and a controlelectrode, a connection from said control electrode of said control tubeto the output circuit of said amplifier tube, an impedance in the platecircuit of said control tube whereby the plate potential thereofdecreases with increase of space current in said control tube, and aconnection from said anode to a control electrode of said amplifier tubeand to said rectifier tube for applying a blocking bias thereto.

4. In radio receiving apparatus, in combination, at least one amplifiertube connected for amplification of incoming signals at super-audiblefrequency, a thermionic rectier supplied by said amplifier tube, aconnection between said rectifier and a control electrode of saidamplifier tube for applying rectified voltage thereto as a bias toregulate the gain thereof inversely with the strength of incomingsignals, an additional rectifying element, a connection between saidadditional rectifying element and the output of said amplifier, acontrol element comprising an anode, a cathode, and a control electrode,means for applying voltage from said additional rectifier element tosaid control element in a manner to cause cut ofi' of plate current insaid control element, anda connection from said anode to a controlelectrode of said amplifier tube to cause` vblocking and unblocking ofsaid amplifier at different values of incoming signal respectively.

5. In radio receiving apparatus, in combination, at least one amplifiertube connected for amplification of incoming signals at super-audiblefrequency, a rectifier supplied by said amplier tube, a connectionbetween said rectifier and a control electrode of said amplifier tubefor applying rectied voltage thereto as a bias to regulate the gainthereof inversely with the strength of incoming signals, an additionalrectifying element having an impedance in circuit therewith, aconnection between said additional rectifying element and the output ofsaid amplier, said connection including said impedance, a controlelement comprising an anode, a cathode and a control electrode, theinput circuit of said control element including at least a portion ofsaid impedance, and a connection from said anode to said first rectiiierto cause blocking and unblocking of said rectifier at different valuesof incoming signal.

6. In radio receiving apparatus, in combination, at least one amplifiertube connected for amplification of incoming signals at super-audiblefrequency, a rectifier supplied by said amplifier tube, a connectionbetween said rectifier and a control electrode of said amplifier tubefor applying rectified voltage thereto as a bias to regulate the gainthereof inversely with the strength of incoming signals, an additionalrectifying element having an impedance in circuit therewith, aconnection between said additional rectifying element and the output ofsaid amplifier, said connection including said impedance, a controlelement comprising an anode, a cathode, and a control electrode, theinput circuit of said control element including at least a portion ofsaid impedance, and a connection from said anode to a control electrodeof said amplifier tube to cause blocking and unblocking of saidamplifier at different values of incoming signal.

7. In radio receiving apparatus, in combination, at least one amplifiertube connected for amplification of incoming signals at super-audiblefrequency, a. rectifier supplied by said amplifier tube, a connectionbetween said rectifier and a control electrode of said amplifier tubefor applying rectified Voltage thereto as a bias to regulate the gainthereof inversely with the strength of incoming signals, an additionalrectifying element having an impedance in circuit therewith, aconnection between said additional rectifying element and the output ofsaid amplifier, said connection includingsaid impedance, a controlelement comprising an anode, a cathode and a -control electrode, theinput circuit of said control element including at least a portion ofsaid impedance, and a connection from said anode to a control electrodeof said amplifier tube and to ysaid first rectifier to cause blockingand unblocking of said amplifier and rectifier at different values ofincoming signal.

HULBURT C. TITTLE.

